1. Field of the Invention
The present invention relates to a torque control device applied to, for example, a screw tightening device and a method for controlling the same, the torque control device tightening connecting members by application of torsional torque.
2. Description of Related Art
It is an object of a known art, for example, Japanese Unexamined Patent Application Publication No. 10-080872, to reduce the overshoot of tightening torque.
A current limiting unit monitors the value of current supplied from a current amplifier to a motor and performs interrupt control of a current reference unit so that the value of current does not exceed a current limit value input to the current limiting unit from a general reference unit.
The current limit value in the current limiting unit is determined by iL(t)=iT·exp{−v(t)/K}, where iL(t) is the current limit value at a time t, iT is the value of current for producing target torque, v(t) is motor rotational speed at the time t, and K is a constant.
The current limit value increases and decreases in response to the motor rotational speed. As a predetermined value of driving current corresponding to the target tightening torque is approached, the value of the driving current gradually increases, and thus the tightening torque also gradually increases, so that the tightening torque gradually reaches the target tightening torque without exceeding the target tightening torque. That is, a screw member is appropriately tightened with predetermined torque without the overshoot of the rotational torque of a motor. Thus, the overshoot of tightening torque can be reduced.
Moreover, for example, it is an object of Japanese Unexamined Patent Application Publication No. 6-182638 to improve the accuracy of screw tightening torque by preventing an actual torque value from being irregular, for example, sharply increasing, due to a change in friction between a screw and a workpiece, lost motion in a mechanical transmission system such as a motor shaft and a speed reducer, and the like.
A torque sensor that detects rotational torque is provided. The detected torque value is fed back, and a torque reference is output to a current driven driver so as to perform control so that motor rotational torque reaches a set torque value, and when the detected torque value reaches a target torque value, a current reference from the current driven driver is turned off to stop a motor.